Retinoscopes are used to obtain objective indications of any refractive errors in eyes. To that end a retinoscope projects a small patch of light onto the fundus of an eye under examination. The user of the retinoscope (i.e. the person conducting the examination) then moves the device so as to move the emitted beam in a given direction, and views the direction in which the patch appears to move after having been refracted and reflected by the eye under examination. The direction of the apparent movement of the patch, relative to the actual movement of the retinoscope, enables the user to determine whether the eye under examination is long-sighted or short-sighted. The user can then quantify the refractive error of the eye under examination by interposing correction lenses of various strengths between the eye under examination and the retinoscope until the user finds a correction lens for which movement of the retinoscope causes substantially no apparent movement of the patch of light. This is referred to the "neutral point" of the eye under examination, and the power of the correction lens that achieves this is indicative of the refractive error.
A conventional spot retinoscope has a light source, such as a light bulb with a conventional helical filament, light from which passes through a focusing lens system, and is then reflected by an angled, semi-silvered or apertured mirror into the eye under examination. An annular stop is interposed between the lens system and the light source, and the shadow cast by the stop defines the boundary of the substantially circular patch of light projected into the eye under examination. The image of the patch of light projected into the eye is viewed by the user from the opposite side of the mirror.
Known spot retinoscopes also have a facility for moving the lens system so as to alter the divergence of the beam of light being directed into the eye under examination, so as to enable the user to alter the size of the projected spot. Large refractive errors cause slow movements of the reflex which are difficult to detect if the spot is large. By reducing the spot size, the movement is made easier to detect. Small refractive errors cause fast movements and are more easily detected when the spot is large.
However, even with a large spot, it can be difficult to determine whether the neutral point has in fact been reached, or whether there is a small amount of movement of the projected spot. Thus, errors in the determination of refractive error can occur if the user of the retinoscope believes the neutral point has been reached, whereas in fact movement of the retinoscope does cause a small undetected amount of movement of the projected spot.